It is a well-established concept in engine design that an increase the performance of an internal-combustion, reciprocating engine can be achieved through various improvements in the exhaust system. One way to improve an exhaust system has been to utilize pressure variations developed within an exhaust system, typically through changing backpressure and resonance, to supplement the control of gases moving within the engine and its exhaust and thereby modify the pressure and pressure waves to improve engine performance. An exhaust system may develop some rather substantial pressures that are both positive and negative with reference to the ambient and these pressures may be effectively harnessed to accomplish more desirable movement of gases through the engine.
It has also been determined that the design considerations for an exhaust system include the operating speed of the engine. Generally, the significance of speed may be somewhat more apparent with the recognition that the exhaust system receives gas pulses, the frequency of which is directly related to the operating speed of the engine.
This is especially true of a two-stroke engine, such as a motorcycle engine or glow ignition two-stroke engine, a type typically found in hobby craft. The two-stroke engine has a single “breathing” cycle, wherein the exhausted gasses pass out of the cylinder and fresh air-gas mixture is taken into the cylinder simultaneously. Essentially, it is desirable to provide a negative pressure at the engine exhaust port during the interval when both the exhaust and intake ports are fully open, so as to more effectively draw a charge of fresh air-gas mixture into the cylinder. Subsequently, as the exhaust port closes it is desirable to provide a positive pressure to restore and maintain the fresh charge of airgas in the cylinder and initiate compression.
Previous efforts at exhaust systems have attempted to provide such pressure variations; however, these efforts have all fallen short due to the complex design considerations. These solutions suffer from significantly increased sizes and weights, characteristics that detract from the performance gains that such solutions provide. Moreover, the vast majority of previous designs do not provide for adequate and convenient adjustment of tuned length. Instead, these designs rely on inaccurate mechanical and manual controls or limited automatic controls, if any at all, that cannot make adjustments during a race or active use.
Generally, applications for exhaust systems for such engines involve a demand for high performance yet, are so varied that flexibility is required. This is particular significant as the size of the engine decreases, e.g., two cycle motorcycle engines versus glow ignition two stroke engines. As the lengths of the acceptable variations and degree of control over these variations moves from the scale of centimeters to millimeters, accurate control becomes increasingly important, as does reducing size and weight. None of the existing designs has been able to provide a satisfactory combination of these important characteristics.
For example, U.S. Pat. No. 2,459,918 to Chester shows a size adaptation system, suitable for use in adapter exhaust pipe extensions to automobile exhaust or tail pipes of different sizes. The structure involves a tubular member, which is slidingly fitted to the exhaust pipe and is equipped with a spring for resisting its forward sliding movement. This design simply provides for movement of the pipe length relative to a bumper, as a means of protecting the pipe. It does not provide for adjustment, much less quick adjustment of the tuned length of the exhaust system. Nor does it provide for manipulation of the convergent and divergent portions of an exhaust in unison or variation in the geometry of such sections to vary either the angle of expansion or the angle of convergence in these sections.
In U.S. Pat. No. 3,703,937 to Tenney, an expansion chamber exhaust system for two-cycle engines with a valve that shifts position from a low RPM position to a high rpm position to provide a positive pressure wave in the exhaust chamber. This adjusts the volume of the chamber and the characteristics of the pressure wave to suit set RPM characteristics. However, the additional components add significant weight to the overall system and require modifications to provide a dual path exhaust chamber. Moreover, this system is overly complex, requiring switching between low and high RPM chamber flows. It is also limited in its RPM response, therefore limiting its overall performance. It also fails to change tuned length. Finally, the reference does not teach nor suggest the movement of both the header or divergent and belly or convergent sections or variation of the relative geometries to vary either the angle of expansion or the angle of convergence in these sections.
Another attempt is provided in U.S. Pat. No. 3,726,092 to Raczuk, which shows an exhaust system for a combustion engine, having an exhaust port with a cylindrical length. In the second embodiment, the cylindrical length has a generally conic convergent section coupled to it and contains a generally conic divergent section slidably received within it at one end. A spring is used to urge the internal conical divergent section along the cylindrical length toward the engine to vary the size of the exhaust. A manual actuator pushes the spring and, thereby, the internal conical divergent section down various positions in the cylindrical length toward the convergent section for different operating speeds of the engine.
Several problems arise in the operation of such a device. For instance, the operator needs to make constant adjustments to the exhaust to change its length. This can tax the ability of the operator to simultaneously change the exhaust length and control the vehicle. Moreover, there are significant problems in using this on small scale vehicles, such as hobby craft. In many instances the operator of these types of vehicles is not traveling on the vehicle and operator adjustment is thus impossible. This design also suffers from significant additional weight due to the control mechanism and cannot be accurately adjusted across a wide range of RPMs, just those for which the controller is pre-positioned.
U.S. Pat. No. 3,969,895 to Krizman describes a power control valve attachment provided for assembly on an existing two-cycle engine exhaust system. The system increases back pressure by providing a perforated section with an end cap that is held within the exhaust pipe. The relative pressure from the engine pushes against the end cap, extending the perforated section out from the exhaust pipe. A spring prevents the system from falling out of the exhaust during operation. In this design, the tuned length of the exhaust system is not truly adjusted. Only the backpressure within the system is increased by obstruction with the end cap. Thus, performance gains are marginal and tuned length is not adjusted over a range of RPMs. Furthermore, the relative movement of both a divergent and/or convergent section is not considered in this design.
Similarly, U.S. Pat. No. 5,785,014 to Cornwell provides for movement of the exhaust controlled by exhaust pressure, but it does so by obstructing the flow of exhaust gases in a similar, albeit more complicated, manner as that of Krizman. This and similar patents provide for a variety of components to reduce the flow cross-section of the exhaust and increase backpressure, however, each falls short in that the reduction in the flow of the exhaust drops peak performance and reduces power at the highest RPM levels, specifically the RPM range beyond the peak power level, where exhaust pressures drop and a restriction in the flow or cross-sectional flow area is highly detrimental to performance.
In U.S. Pat. No. 4,715,472 to McKee an adjustable motorcycle muffler with a stationary ring and an adjustable ring at an exit end is provided. The adjustable ring is movable relative to the stationary ring to vary the amount of gas flow. Again, this increases the back pressure but does not provide for adjustment of the tuned length of the exhaust system, thus, performance gains are marginal and tuned length is not adjusted. Furthermore, the design fails to move either a belly or convergent cone or a header or divergent cone section, much less move these elements simultaneously.
The U.S. Pat. No. 5,214,254 to Sheehan discloses a triple cone exhaust for controlling both flow and resonance within the exhaust. The triple cone has a tubular perforated sleeve, a tubular perforated tuning pipe with a conical end, and a reverse cone megaphone enclosure with the inlet and outlet of the exhaust on either end. The orifice size, and thereby the resonance of the system, can be adjusted by turning the sleeve. Again, the tuned length of the system is not adjusted in this solution. Instead the resonance of the pressure wave within the system is adjusted by adjusting the “noise” level created within the exhaust, that is the pressure exerted by sound waves within the exhaust. It does not adjust either the length of a divergent or the convergent cone section.
In U.S. Pat. No. 5,218,819 to Cruickshank provides an exhaust system with a variable volume by displacement of a baffle member in a baffle chamber. The volume of the chamber is increased by a baffle that opens and adds volume to the chamber. Although this may change the pressure wave within the system, it does not provide for adjustment of the tuned length of the system. Additionally, by simply adding volume in this fashion to manipulate the pressure wave it results in a larger overall exhaust, adding weight that detracts from performance gains. In addition, although the overall volume of the exhaust may be varied, the critical parts in developing the pressure waves, namely those elements like the convergent and divergent cones that produce pressure wave reflections, are not being efficiently manipulated. As the convergent and divergent cone sections are not manipulated and the system is utilizing a larger bore exhaust, its performance is diminished significantly.
U.S. Pat. No. 6,520,285 shows an adjustable muffler system for attachment to an engine exhaust system and method of adjusting or tuning the volume level of the sound emitted from an engine muffler. Again, this does not provide for dynamic adjustment of the tuned length of the exhaust system. Instead, it allows for the adjustment of the resonance or sound within the exhaust, which has little functional application to performance improvement. It also fails to manipulate either a divergent or convergent cone section to do this.
Thus, these prior attempts are inadequately addressing the problem of quickly and conveniently adjusting the tuned length of an exhaust system to provide for increased performance across a wide range of applications and engine speeds. Additionally, the heretofore known devices fail to provide for modular construction that would speed repair and replacement in situations such as racing of hobby craft and the like. In these situations rapid replacement of a modular component during competition can mean winning or losing the race. Moreover, quick adjustments during pit stops and the like would also need to be facilitated. Consequently, a need exists for an improved, more flexible modular and quickly adjustable tuned exhaust system that may be more uniformly used on a wide variety of small engines and which is capable of accomplishing improved operating performance at various engine speeds while allowing for rapid replacement, adjustment or replacement and adjustment in a vehicle.